Scrub typhus, also referred to as chigger-borne rickettsiosis, mite-borne typhus, Japanese river fever, tropical or rural typhus or tsutsugamushi disease is an acute, febrile disease caused by infection with Orientia (formerly Rickettsia) tsutsugamushi. It accounts for up to 23% of all febrile episodes in endemic areas of the Asia-Pacific region (1). The disease is characterized by a rise in body temperature, skin rash, and severe headaches. This disease may affect the nervous system, with clinical manifestations such as delirium, stupor and muscle fibrillation. The death rate varies from 1 to 30% depending on the virulence of the infecting strain.
Scrub typhus is particularly prevalent in South-East Asia, Korea, Australia, China, Japan, and India. The incidence of disease has increased in some countries during the past several years (40). The causative organism of scrub typhus is transmitted to human through the bite of chigger. These organisms are found throughout the mite's body, with the highest number resides in the salivary glands. When chigger feeds on mammals, including cattle, rodents or humans, the disease causing organisms are transmitted from the mite to a vertebrate host (subject). Scrub typhus infections are usually found in people engaged in activities that bring them inadvertently in contact with mite-infested habitats or any vertebrate host-carrier of these anthropods. These hosts may include domesticated or non-domesticated animals, such as cattle or rodents. These hosts may be carrying mites which have not begun to feed on them. In this case, the live mites can be transferred from the vertebrate host to people. Individuals particularly susceptible include butchers, meatworkers, animal-farm workers, and others engaged in outdoor activities. These persons could be infected by coming into contact with the mite-carrying animals. Additionally, rodents are capable of carrying and spreading infected mites to people in populated areas. Larval Leptotrombidium mites feed on vertebrate hosts. The larval mites acquire O. tsutsugama through their female parent. This type of pathogen reception is called “transovarial transmission.”
Once transmitted to the host, the organism incubates for about 10 to 12 days before the onset of illness. Five to eight days after infection, a dull red rash and/or eschar may appear on the body, especially on the trunk. If left untreated, O. tsutsugamushi can cause up to 35% mortality. A recent report from India documented 17% case fatality rate (3). At the present time, no vaccine is available for protection against scrub typhus. Recent evidence of antibiotic resistance of O. tsutsugamushi further emphasizes the need for a scrub typhus vaccine (13, 14).
Diagnosis of scrub typhus is generally based on clinical presentation and patient history. However, differentiating scrub typhus from other acute tropical febrile illnesses such as leptospirosis, murine typhus, malaria, dengue fever, and viral hemorrhagic fevers can be difficult due to similarities in signs and symptoms. Highly sensitive polymerase chain reaction (PCR) methods have made it possible to detect O. tsutsugamushi at the onset of illness, when antibody titers are not high enough to be detected (41, 44, 48). PCR amplification of the 56 kDa protein gene has been demonstrated to be a reliable diagnostic method for scrub typhus (41, 46). Furthermore, different genotypes associated with different Orientia serotypes could be identified by analysis of variable regions of this gene without isolation of the organism (41, 42, 43, 46, 49). However, gene amplification often requires sophisticated instrumentation and expensive reagents, which are generally not available in the rural medical facilities. Current serodiagnostic assays, such as the indirect immunoperoxidase (IIP) test, the indirect immunofluorescent antibody (IFA) test or the microimmunofluorescent antibody (MIF) test, require propagation of rickettsiae in infected yolk sacs of embryonated chicken eggs or antibiotic free cell cultures (51).
Currently, the only commercially available dot-blot immunologic assay kit, DIP-S-Ticks Scrub Typhus Diagnostic Test Kit (Panbio, Queensland, Australia) requires steps of growing the disease causing organisms in tissue culture, purifying of the organisms using Renografin density gradient, and the extraction of the whole cell antigen (50). However, only a few specialized laboratories have the ability to culture and purify O. tsutsugamushi since these procedures must be carried out under biosafety level 3 (BL3) requirements. Furthermore, large-scale growth and purification of the Orientia are prohibitively expensive. Therefore, the availability of recombinant rickettsial protein antigens that can be produced and purified in large amounts and have similar sensitivity and specificity to Orientia-derived antigens, would greatly reduce the cost, transport, and reproducibility problems presently associated with diagnostic tests of scrub typhus.
O. tsutsugamushi also exhibits considerable strain variation (15-18). Homologous protection developed from natural infection persists for at least one year, but heterologous protection may remain for less than six months (19, 20). Both humoral and cell-mediated immune responses are important in protective immunity against scrub typhus (21-25). Prior vaccine development efforts using whole organism suggest that a scrub typhus vaccine is possible. Effective vaccination in mice has been achieved with a biovaccine comprising a single dose of live organisms in combination with chloramphenicol or a vaccine comprising gamma-irradiated live organisms (26, 27). Immunization of volunteers with live vaccine in combination with chloramphenicol prophylaxis elicited immunity comparable to that of natural infection (19). Although a recent report suggested that long-term adaptation in egg-yolk sac has increased the yield of Orientia (28), considerable difficulties still exist in mass production of purified O. tsutsugamushi and in retaining its stability upon storage. Consequently, whole cell vaccine products are unlikely to be economically feasible or suitable for manufacturing with current Good Manufacturing Practices Act standards of purity, potency, and lot-to-lot consistency. Furthermore, not every component in the whole cell antigen is protective. It has been demonstrated that the 22 kDa antigen not only did not provide any protection, but also inhibited the protection provided by other antigens (29). Therefore, it is essential to develop a subunit vaccine composed of genetically engineered antigens which are capable of inducing protective immunity in human subjects.